Definitely not too big for HO, and very common in the '30s, although maybe not without visible seam lines. You might be better off to use these a bit back from the foreground, although as scenery, they will tend to fade into the background anyway if you have more detailed stuff in the foreground.
I assume that these are metal, so you'll probably not want to add much 3-D detail to them. However, an easy way to add a bit of detail is with paint. Lay out strips of masking tape on a sheet of glass, and using a new blade in your X-Acto, trim the strips to a suitable width, say 8' or 10', and to a length equal to about 1/3 or 1/4 of that of the circumference of the tank. Starting at the top of the tank, apply each of the 3 or 4 pieces around the circumference of the tank, leaving 3" or 4" scale between each "panel". Work your way to the bottom of the tank, leaving a similar space between each subsequent ring of tape and making sure to stagger the vertical joints on alternating rings. The effect should look like giant bricks around the tank, with the vertical joints of every other ring lining up. Don't worry if the bottom row of tape works out to be less than full height, as lower tank rings were often shorter in height than those of the rest of the tank.
Now you need to decide how "old" you want the tank to appear: if it's fairly new, or well-maintained, choose a colour similar to the base colour of the tank, but slightly lighter or slightly darker. The ideal scenario would be to choose a tank colour first, divide it into two different jars, and then darken one with a little black or dark brown paint. Pick either one to paint the body of the tank, then use the other (lighter or darker) version for the next step, which is to apply the second colour to the masked tank. Because the colours are similar, but not exactly the same, the effect when you remove the masking tape will be of riveted panels making up the tank. However, before you remove the tape, there is an additional step you can do to enhance the panel effect and make the tank appear a bit older. Mix up a batch of your favourite rust colour, keeping it more brown than orange, then thin it severely (about 75% - 80% thinner). Very lightly spray all of the vertical "seams", then remove the bottom band of masking tape. With your airbrush almost parallel to the side of the tank, and the tip close to the edge of the lowermost strip of masking tape, and pointing towards the base of the tank, spray around the circumference of the tank. Remove the next lowest strip of tape, and repeat until you reach the top. If you use a light touch, the effect should be of a tank composed of steel panels riveted together, with a bit of rust bleeding down the sides from each of the horizontal joints. By starting the weathering process at the bottom, any slight overspray will simply make it look as if the rust has been washed down the sides by rain and gravity.
If you want to apply lettering, like product logos, etc., it's probably best done before doing any of this work, including the seam lines.
I'm not sure if this picture is much help as an example, as my computer monitor doesn't show much in the way of red tones. While my tank is styrene, wrapped around a cardboard tube, and has had rivets embossed on it, the weathering effect should be very subtle: you can get away with a little heavier weathering because you don't have actual "rivets", but keep it light. If your first effort isn't to your liking, you can always do a second application.
Good luck if you attempt this procedure, which is probably do-able with spray cans, but much easier with an airbrush. And if the whole thing seems to be too much bother, your tanks still look fine "as-is".
(Edit) I just viewed the photo here: on my monitor, the caboose, which is painted with Floquil Caboose Red, appears dark brown, with a definite greyish cast. I can only hope that the true appearance is closer to what I see in the layout room.
Wayne